1. Field of the Invention
This invention relates to a light activated silicon switch (LASS).
2. Description of the Prior Art
In order to actuate a LASS, a large amount of light must be delivered to the proper regions inside the device. In order to accomplish this it has been the practice to direct a beam of light on to the surface of the silicon wafer. Typically, the beam is sent in a perpendicular direction to the wafer surface or at Brewster's angle in order to minimize the surface reflection loss.
There are some rather severe disadvantages with these prior art techniques for light coupling to the LASS. Typically, the light is introduced through a central aperture, or one or more rectangular slots arranged in the cathode electrode. Thus the light is introduced into a relatively compact area. It is characteristic of the LASS that current conduction spreads slowly from the region initially turned on as determined by the area exposed to light. Thus it takes a relatively long time for conduction to spread from the light area to cover the entire useful area of the device.
From the illuminated area, the shortest electrical path would be in a direction toward the aperture in the cathode electrode, but obviously, since this presents no electrical contact, the path to the cathode must be longer. This much of the initial current passes through long, relatively high resistance paths in the silicon before it can enter the metal contacts around the aperture. This high resistance causes rapid heating at the beginning of current flow, and limits the amount of current that can be tolerated without device failure.
Further, with the single aperture, or the one or more rectangular slots in the cathode electrode, the heat generated in the silicon wafer in the region where the light impinges, must seek a longer thermal path to those parts of the cathode electrode which are sill intact. There is of course still a thermal path to the anode electrode which also serves as a heat sink. As a result, thermal considerations place an upper limit on the starting current capacity for the device.